GB plate:
https://c7743.ssl.cf2.rackcdn.com/Im...con/RNF388.jpg
UK (registration) plate front:
http://www.triple-c.com/images/lp_acrylic_wh.jpg
UK (registration) plate rear:
http://www.triple-c.com/images/lp_acrylic_yl.jpg
or, more appropriately for most of us:
http://www.licenseplates.tv/images/eugbqby.gif
The front (black on white) long ones are easy to find, the rear (black on yellow) long ones are easy to find, the rear (black on yellow) square-ish ones are hard to find, and Les just linked to the really old style.